One important result of the recent, increased social awareness of the needs and rights of handicapped persons has been the development of telecommunications devices for the deaf (TDD). Such devices, typically comprising a battery-powered, microprocessor-controlled teletypewriter with moving visual display, are acoustically coupled to conventional telephones to permit hearing-impaired persons to communicate readily and conveniently with others.
Special provisions must be made to alert hearing-impaired persons of incoming calls. One known ring indicator can be connected to any room lamp to cause the lamp to flash on repeatedly when ringing voltage is present on the telephone line. Further, if the lamp is being used to illuminate the room, the indicator will cause the lamp to flash off repeatedly to alert the hearing-impaired person. However, from a human-factors viewpoint, such an indicator is highly undesirable when the lamp used for alerting is also used to illuminate the room because the people in the room will be subjected to darkness during the active part of each ringing cycle. Accordingly, it is an object of the present invention to provide a reliable, inexpensive ringing detector which can be employed in conjunction with a lamp used for room illumination without annoying light interruptions during ringing.